Method of preparing circuit boards comprising inductors

ABSTRACT

A ferrite inductor is coated with fluorocarbon particles prior to attachment to a printed circuit board.

BACKGROUND OF THE INVENTION

In the manufacture of printed circuit boards it has been necessary toconnect coils for inductors and transformers to the other boardcircuitry by physical attachment of the coils to the boards. In presentpractice the coils have ferrite cores, and, where tuning is required,the cores have tunable ferrite slugs. As the final step in the physicalmanufacture of a printed circuit board it is customary to apply alacquer-like protective layer, known as a conformal coating, over theentire surface of the board. This is done by dipping the completed boardin an acrylic solution. On a production basis printed circuit boards arecarried on a conveyor through a tank of the solution and through an airdrying station, following which they are stored for testing and finaltuning of any tunable inductors. It has been found, however, that thefinal tuning is obstructed, in many instances, by the conformal coatingwhich fills the fine threads of the tuning device and/or binds the slugin its off-tune position.

Although the conformal coating operation does apply a protective layerto the wire of the induction coil, it does not encapsulate the coil to adegree that moisture is excluded from the turns, and changes in moisturecontent can cause variations in the inductive reactance of inductors onconventional printed circuit boards. Sealing of the core containing thewire coil by means of silicone rubber has been practiced but thisexpedient has the disadvantages of high cost and the possible entrapmentof any moisture that may find its way into the coil area.

SUMMARY

I have invented the method of preparing a printed circuit board that hasattached to it a member that includes an induction coil which may have aferrite core and may comprise an adjustable slug for the core. My methodcomprises the steps of coating the member with fine particles of afluorocarbon polymer and then coating the entire board, including themember with a conformal coating which may comprise an acrylic resin.Preferably the member is immersed in a suspension ofpolytetrafluoroethylene particles in a liquid carrier such as 1,1,2trichloro 1,2,2 trifluoroethane, and a small proportion, relative to theproportion of polytetrafluoroethylene, of polyperfluoropropylene iscomprised in the suspension.

I have invented an inductor comprising a coil of insulated wire, aferrite core, a helically threaded tubular surface associated with thecore, an adjustable ferrite slug, a helically threaded cylindricalsurface, fitting the tubular surface, associated with the slug, and afine layer of particles of fluorocarbon polymer, such aspolytetrafluoroethylene polymer which may advantageously be admixed witha small proportion of polyperfluoropropylene polymer, coating the wireand the threaded surfaces, which may be polymeric. A printed circuitboard of my invention comprises at least one inductor which may comprisea ferrite core and an adjustable slug and a plurality of conductingsurfaces and non-inductor circuit elements. The conducting surfaces andnon-inductor circuit elements are covered with an adherent conformalcoating while the inductor is free from adherence to the conformalcoating and has its surfaces coated with a layer of fluorocarbonparticles such as particles of polytetrafluoroethylene.

I have invented the improvement in litz wire coils where the litz wirecomprises a large plurality of fine insulated metal filaments coveredwith a fibrous serving of having the serving impregnated with finediscrete fluorocarbon particles such as particles ofpolytetrafluoroethylene.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows steps in an embodiment of the method of my invention.

FIG. 2 shows a section of an inductor comprising the improvement of myinvention.

FIG. 3 shows a plan view of a printed circuit board of my invention.

FIG. 4 shows a section of a litz wire coil encompassing my invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1 my novel method of making printed circuitboards will be described. Conventional printed circuit boards 11 towhich inductors 12 are to be attached are advanced in an assembly linein the usual manner wherein at some station 13 the inductor 12 will beadded to the board. Thereafter the board passes through additionalstations for the addition of further circuit elements and for testing.Finally the completed board reaches a conveyor 16 which carries itthrough a tank 17 of conformal coating solution of which differentformulations are known and used, such as solutions of acrylic resins.Upon leaving the tank 17 the conveyor 16 traverses a distance longenough to allow evaporation of the solvent of the conformal coatingwhereby a protective coating is applied to all exposed surfaces of theprinted circuit board 11. Prior to application of the conformal coatingand conveniently prior to its attachment to the printed circuit board atstation 13 the inductor 12 is passed through steps that are essential tomy method and that are not applied to those elements of the printedcircuit board which do not comprise wire coils or tunable cores. Inthese steps the inductor 12 is immersed in a suspension 18 ofpolytetrafluoroethylene in a volatile carrier. As a carrier I prefer touse 1,1,2 trichloro 1,2,2 trifluoroethane but other volatile fluorinatedand chlorinated hydrocarbons and ketones such as acetone or suitableblends thereof are included within the scope of my invention as acarrier for the suspension 18. The suspension 18 is maintained in a tank19 and a large number of the inductors 12 are conveniently placed in awire mesh basket 21 for immersion in the tank for 10 to 15 minutes.After the period of immersion has been completed the basket 21 is placedin a vacuum chamber 22 whence the volatile carrier is completely removedthrough an exhaust 20 and the treated inductors delivered in anyconvenient manner to the station 13. At station 25 the finished boardsare tested and any adjustments that are found to be necessary are madeat station 30. In commercial production a significant proportion ofinductors has been found to require tuning following the conformalcoating step and in a commercially disadvantageous number of theseproper tuning of prior art inductors has been impossible due, usually,to jamming of the threaded slug, as shall be explained. Where thisoccurs and it is required to remove the inductor from the board andreplace it with another, the cost is excessive. Following myintroduction of a fluorocarbon treatment of the inductors in advance ofthe conformal coating steps fine tuning has been successfullyaccomplished in essentially 100% of the production in which it isnecessary. The suspension in the tank 19 consists of a carrier of 93.9weight percent 1,1,2 trichloro 1,2,2 trifluoroethane, 5.6 weight percentof polytetrafluoroethylene, and 0.5 weight percent ofpolyperfluoropropylene. The particle size of the polyfluorocarbonparticles should have more than half the particles in the submicronrange and substantially all under 10 microns. An actual count of theparticle distribution is shown in the TABLE.

                  TABLE                                                           ______________________________________                                        Fluorocarbon Particle Size Distribution                                       Microns     % Number of Particles                                             ______________________________________                                        less than 0.25                                                                            24                                                                0.25 - 0.5  23                                                                0.5 - 1.0   22                                                                1 - 2       13                                                                2 - 4       10                                                                4 - 6        6                                                                6 - 10       2                                                                ______________________________________                                    

In FIG. 2 I have shown an induction coil 12 of a type for which myinvention has particular applicability. The wire coil 23 is formed of alarge number of turns of a litz wire 24 on a plastic bobbin 26 that isenclosed in a ferrite core 27. The core 27 has two half sections 28, 29that have been cemented together at a seam 31 after the insertion of thefilled bobbin 26. Tuning of the coil is accomplished by means of aferrite slug 32 slidable in a plastic tube 33 having an annularthickened portion 34 with a threaded tubular surface 36. The threadsforming the surface 36 match a threaded cylindrical surface 37 of aplastic plug 38 that is tightly attached to the slug 32. A screw head 39with a slot 41 of the plug 38 enables the slug 32 to be accuratelypositioned within the coil 23 for fine adjustment of the coil impedance.My invention has been successfully practiced where the tube 33 and thesurface 36 are polycarbonate and the plug 38 and surface 37 arepolyacetal. It is not believed, however, that the substance of thesesurfaces is critical to the practice of my invention. Indeed where thethreaded surfaces are cut or formed directly into the ferrite, as isshown in U.S. Pat. No. 3,471,815 or in other ceramics, or plastics such,for example, as nylon, polypropylene, and polyimide my invention may bepracticed advantageously.

When the coil 12 is immersed in the tank 19 a fine deposit ofpolyfluorocarbon particles forms on all the surfaces of the coilincluding the threaded surfaces 36 and 37. The particles also depositupon a surface 42 of the bobbin 26 and an outer surface 43 of the wire24. This is possible in spite of the enclosure of the bobbin 26 withinthe core 27 because the latter is slotted through its outer cylindricalwall 44 at a slot 46 that has the purpose of permitting access to theends of the wires of the coil. It has been common to seal this slot 46with a silicone rubber filling and it is an advantage of our inventionthat it has precluded the need for this relatively expensive sealingstep. The litz wire 24 is made up of a large number, such as 85, No. 44Awg (American Wire Gage) (0.05 mm) metal filaments or wires, 47 (FIG.4), each with an enamel insulation 48 and twisted together in a bunchwith a long right hand lay. The bunch is served with a fine fibrousrayon serving 49 having a left hand lay. The serving 49 is permeable tomoisture which will change, somewhat, the specific inductive capacitanceof the insulation 48. In my method the polyfluorocarbon particlespenetrate the fibers of the serving 49 making it water repellant. Theseparticles are discrete, by which I mean that, although some of them maybe contiguous to each other or form small agglomerates and, due,apparently, to surface forces, will remain in position in the absence ofa substantial washing action, the particles are not bound together or tothe substrata by any organic adhesive or by fusion. To achieve thiseffect and also to penetrate between the surfaces 36 and 37 it isessential that sufficient polyfluorocarbon particles of a submicron sizeshould be contained in the suspension within the tank 19. Although myinvention does have particular advantage for coils made with litz wire,its usefulness is not restricted to such coils and I have found thattunable magnet wire coils also benefit from the new method.

The practice of my invention results in an improved printed circuitboard of FIG. 3 having circuit elements indicated diagramatically by thecircles 52 interconnected by conducting surfaces 53. The board alsoincludes one or more of the inductors 12 the surfaces of which, alongwith all the other board surfaces, have been treated to receive aconformal coating. However the conformal coating is adhesive to thesurfaces of the board 51, elements 52, and conductors 53 but not to thesurfaces of the inductor 12 which have been polyfluorocarbon coated.

In the selection of a suitable polyfluorocarbon for the practice of myinvention I prefer the use of polytetrafluoroethylene as the principalactive polymer. Other polymeric fluorocarbon particles such, forexample, as chlorotrifluoroethylene and its copolymers with vinylidenefluoride, poly(ethylene-chlorofluoroethylene), fluorinatedethylene-propylene and the homopolymer of vinylidene fluoride may alsobe used within the scope of my invention.

The foregoing description has been exemplary rather than definitive ofmy invention for which I desire an award of Letters Patent as defined inthe following claims.

I claim:
 1. The method of preparing a printed circuit board havingfastened thereto a member including an induction coil having a ferritecore and a slug for adjusting the position of said ferrite core withrespect to said induction coil, said method comprising the steps of:A.applying a conformal coating to said printed circuit board includingsaid member; B. prior to said application of said conformal coating,coating the surfaces of said member with fine particles of afluorocarbon polymer which prevents adherence of said conformal coatingto said surfaces of said member so that said ferrite core is adaptedfreely for movement; and, C. moving said ferrite core to change theimpedance of said induction coil after the application of said conformalcoating.
 2. The method of claim 1 wherein said slug comprises a threadedpolymeric element.
 3. The method of preparing a printed circuit boardhaving fastened thereto an electrical member having movable means forvarying the impedance of said electrical member, said method comprisingthe steps of:A. coating the surfaces of said electrical member andmovable means with particles of fluorocarbon polymer; and thereafter, B.applying a conformal coating to said printed circuit board includingsaid electrical member and movable means, said conformal coating beingnon-adherent to said surfaces of said electrical member and movablemeans because of the coating of step A thereby to permit unrestrainedmovement of said movable means whereby the impedance of said electricalmember may be varied; and thereafter, C. moving said movable means. 4.The method of claim 3 comprising the step of fastening said member tosaid board, said surfaces being coated with said particles prior to saidfastening.
 5. The method of claim 3 wherein said member comprises aferrite core for said coil.
 6. The method of claim 4 wherein said membercomprises a ferrite core for said coil.
 7. The method of claim 3 whereinsaid conformal coating comprises an acrylic resin.
 8. The method ofclaim 7 wherein said conformal coating step comprises dipping said boardinto a solution of said resin.
 9. The method of claim 4 wherein saidfluorocarbon coating step comprises immersing said member in asuspension of said particles.
 10. The method of claim 9 comprising thestep of vacuum drying said member to remove a volatile component of saidsuspension.
 11. The method of claim 3 wherein said polymer ispolytetrafluoroethylene.
 12. The method of claim 4 wherein said polymeris polytetrafluoroethylene.
 13. The method of claim 5 wherein saidpolymer is polytetrafluoroethylene.
 14. The method of claim 6 whereinsaid polymer is polytetrafluoroethylene.
 15. The method of claim 9wherein said polymer is polytetrafluoroethylene.
 16. The method of claim15 wherein said polytetrafluoroethylene particles are suspended in aliquid carrier comprising 1,1,2 trichloro 1,2,2 trifluoroethane.
 17. Themethod of claim 15 wherein said suspension comprises particles ofpolyperfluoropropylene, the proportion of said polyperfluoropropylenebeing small relative to the proportion of said polytetrafluoroethylene.